Many electronic devices in motor vehicles operate at voltages less than the vehicle's battery voltage. By way of example, an audio amplifier of an entertainment system, might require a nominal power supply voltage of 5 volts. If the amplifier's transistor output is connected to vehicle speakers by wires that are routed along or adjacent to other conductors that can carry the vehicle's battery supply voltage, a possibility exists that the battery supply voltage carried in adjacent wires might be inadvertently connected or “shorted” to the output stage of the audio amplifier. When a semiconductor is exposed to a voltage that exceeds its maximum rated voltage, it is often damaged or destroyed.
Some semiconductor circuits can be protected from overvoltage using a Zener diode connected between the semiconductor device and a reference potential. Others may use a protection circuit that disconnects the device when a Zener voltage of a Zener diode is exceeded. A Zener diode, however, will only conduct when an applied reverse bias voltage is great enough to trigger its breakdown, and this voltage cannot be accurately controlled. A Zener diode is, therefore, unable to protect against possible voltages that are less than its breakdown voltage. Another problem with using a Zener diode to protect a semiconductor from overvoltage is that its response time may be slow. It can also interfere with analog and digital signals as well as radio frequency signals output from a semiconductor. A circuit that is able to protect the output stage of a semiconductor from overvoltages without having the problems of a Zener diode or other methods would be an improvement over the prior art.